Conversion of heat to electricity



March 23, 1965 J. CREEDON ETAL 3,175,105

CONVERSION OF HEAT TO ELECTRICITY Filed July 28, 1961 HEAT Q i 7 g 4 RL8 n 1' I? O O D c U U v H M13 INVENTORS,

JOHN E. CREEDON SOL SCHNEIDER 8 MORTIMER H. Z/NN.

A T TORNE Y.

United States Patent 3,175,105 CONVERSION OF HEAT T0 ELECTRIClTY John E.Creedon and Sol Schneider, Little Silver, and

Mortimer H. Zinn, West Long Branch, N..l., assignors to the UnitedStates of America as represented by the Secretary of the Army Filed July28, 1961, Ser. No. 127,747 4 Claims. (Cl. 310-4) (Granted under Title35, U.S. Code (1952), sec. 266) The invention described herein may bemanufactured and used by or for the Government for governmental purposeswithout the payment of any royalty thereon.

This invention relates to the conversion of heat to elec tricity andmore particularly to such conversion by means of electric dischargedevices of the thermionic converter type.

Thermionic converters utilize the Edison effect to convert heat directlyto electricity without the aid of moving parts or chemical reactions.Although these devices have been known for many years, the increasedneed for lightweight, reliable power supplies for satellites andportable field equipment has revived interest in this means forgenerating electricity.

A thermionic converter is essentially a diode type discharge devicecomprising an electron emitting cathode and an electron collectinganode. If the work function of the cathode is higher than that of theanode, power will be delivered to an external load circuit on theapplication of sufiicient heat to the cathode to cause thermionicemission. Such an elementary thermionic converter is inefiicient becauseonly a small percentage of the emitted electrons reach the anode, therest falling back to the cathode. The reason for this inefficiency istwo-fold (1) no ac celerating anode voltage is available for attractingthe electrons and, (2) the presence of a cloud of free electrons in thedischarge space creates an electric field tending to repel electronsbeing emitted from the cathode. This phenomena is known as the spacecharge effect. The result is that emitted electrons must climb up apotential hill in order to reach the anode and only those electronswhich leave the cathode at high velocity have sufficient kinetic energyto do this.

Attempts have been made in the past to overcome the space charge effectby such means as extremely small electrode spacing but this results inpractical difliculties because of the close tolerances required.Further, it has been suggested in the prior art to neutralize the spacecharge by projecting an easily ionizable vapor into the discharge space.The vapor molecules, usually those of an alkali metal, are ionized bycontact with the hot cathode. While this method reduces the space chargeeffect, it requires an auxiliary reservoir of alkali metal which must bethermally controlled to produce the desired amount of vapor.

The instant invention provides for eficient neutralization of spacecharge in a thermionic converter by novel and simple means. It istherefore an object of this invention to provide a practical andeflicient source of electrical power of the thermionic converter type.

It is a further object of this invention to provide a novel and usefultwo stage thermionic converter which combines two diverse dischargechambers in a manner which results in a high output power and highCarnot efiiciency.

Other objects and advantages of the invention will become apparent fromthe following detailed description and drawing, which is a pictorialrepresentation of the device.

The drawing shows a two-stage thermionic converter comprising twodischarge chambers 5 and 11, formed by three parallel electrodes 3, 7and 15, and insulated side walls 4. Electrode 3 forms the cathode of thefirst stage iatented Mar. 23, 1965 of the converter and the top surfaceof intermediate elec trode 7 forms the anode thereof. The lower surfaceof 7 forms the cathode of the second stage of the converter and thelower electrode 15 is the anode thereof. A source of heat is applied toelectrode 3, resulting in a negative temperature gradient from the topto the bottom of the device. If electrode 3 is selected to have a highwork function, electrode 15 a low Work function and electrode 7 a workfunction intermediate the other two, power will be delivered to externalload R Alternatively, the upper and lower surfaces of intermediateelectrode 7 may be given different Work functions by depositingactivating agents such as thorium or the oxides of barium and strontiumon the electrode surfaces. The work functions are chosen to give maximumefficiency at the operating temperature.

Each of the chambers of the device is provided with self-contained meansfor neutralizing space charge. The principle of operation depends on thedirect production of neutralizing ions by means of an ionic cathodewhich emits a copious supply of ions when heated. These ion emitters aresimilar to conventional electron emitting cathodes and can becharacterized by the Richardson equation. It has been found that aneificient ion emitter consists of one of the metallic alkalialumina-silicates. At practical operating temperatures, ion emission ofone to ten milliamperes per square centimeter can be obtained from thesecompositions. The particular alkali used determines the type of ionemitted. Although each positive ion emitted carries the same charge as asingle electron. it is capable of neutralizing the field due to severalhundred electrons because it remains in the discharge space much longerthan does an electron because of its relatively large mass and resultantlow velocity.

Referring again to the drawing, both surfaces of intermediate electrode7 are provided with a plurality of beads, 9 and 13, of the ion emittingmaterial specified above. The use of a plurality of small beads of ionemitting material, distributed over both surfaces of the intermediateelectrode, assures an even distribution of neutralizing positive ions inboth discharge chambers. While the ionic cathode may be located anywherewithin the chambers, it has been found that the temperature of thecenter electrode is in the best range for thermionic ion emission. Thetotal ion emitting area is chosen to provide sufiicient ions toneutralize the space charge and to replenish ion losses. Ion loss occursdue to recombination caused by electron-ion collisions and due toabsorption by the electrodes and chamber walls. In accordance with onefeature of the invention, ion losses in the upper chamber are reduced byselecting a type of ion and a cathode with a work function which permitscontact ionization of any ions or re-combined atoms which strike thecathode. If the ionization potential of the neutralizing ions is lessthan the Work function of the cathode 3, contact ionization is possibleprovided the cathode temperature is sufiiciently high. Under theseconditions, any ions or re-combined atoms which strike cathode 3 will bereturned to the discharge space as ions, thus reducing ion losses. Dueto the temperature gradient in the device, the cathode of the lowerchamber will ordinarily not be at sufiiciently high temperature topermit contact ionization.

Other means may be provided for reducing ion loss, thereby reducing theamount of ion emitting material required on the intermediate electrode.For example, the alumino-silicates of lithium have been found to providea very stable ion emitter. Further, lithium has the lowest atomic weightand the highest ionization potential of the five metallic alkalis.Accordingly, if one or both of the discharge chambers are filled withone of the alkali vapors with a higher atomic weight and lowerionization potential than lithium, the positive charge on the emittedlithium ions will be transferred to the vapor molecules, therebycreating heavier, slower moving ions which are more effective inneutralizing space charge, as explained above. For example, if cesiumvapor with an atomic weight of 133 and ionization potential of 3.9 ev.is used as the gaseous atmosphere, the cesium ions produced will beabout 19 times heavier than the lithium ions emitted by the intermediateelectrode, due to the great difference in the atomic weights of thesetwo elements.

In summary, it is seen that each discharge chamber is provided with asimple and effective means for overcoming space charge effect Withoutthe disadvantages of the prior art devices. While each chamber of thetwostage device illustrated can be used separately, the dualconstruction shown takes advantage of the inherent temperature gradientof the device to produce a greater output with only a slight increase inthe complexity of the device. Since the upper chamber operates at hightemperature, the lower chamber, in effect, uses what would otherwise beWaste heat from the upper chamber.

While a preferred embodiment of the invention has been shown anddescribed many modifications may be made by those skilled in the artwithout departing from the spirit of the invention. For example, whilethe ion emitters have been shown as beads of material attached to theelectrode surface, it is obvious that the ion emitters and theintermediate electrode may be made integral by mixing the required ionemitter and electrode materials in powder form together with a suitablebinder, molding the mixture into the required shape and sintering.

Accordingly, the invention should be limited only by the scope of theappended claims.

What is claimed is:

1. A two-stage space charge neutralized thermionic converter comprisingfirst and second discharge chambers formed by first, second and thirdparallel electrodes, said first discharge chamber formed by said firstand second electrodes as the cathode and anode thereof and said seconddischarge chamber formed by said second and third electrodes as thecathode and anode thereof, means to apply heat to said first electrode,said electrodes having progressively lower work functions in thedirection of heat flow, said heat being suflicient to cause substantialthermionic electron emission from both of said cath odes, said secondelectrode having a plurality of uniformly distributed beads of ionemitting material deposited on each surface thereof whereby said beadsemit positive alkali ions for neutralizing space charge, said firstelectrode having a work function greater than the ionization potentialof said alkali ions, and an electrical load device connected externallybetween said first and third electrodes.

2. The device of claim 1 in which said ion emitting material comprises ametallic alkali alumino-silicate.

3. A thermionic converter comprising two spaced, generally parallelelectrodes, one electrode having a higher work function than the other,means to apply heat to the electrode of higher work function, an ioniccathode composed of an alumino-silicate of lithium disposed within saidthermionic converter, said heat being sutficient to cause substantialthermionic electron emission from said electrode of higher work functionand substantial thermionic ion emission from said ionic cathode, saidconverter filled with an alkali vapor of atomic weight greater thanlithium and ionization potential less than lithium.

4. A thermionic converter comprising two spaced electrodes, oneelectrode having a higher work function than the other, means to applyheat to the electrode of higher work function, an ionic cathode disposedwithin said thermionic converter, said heat being suflicient to causesubstantial thermionic electron emission from said electrode of higherwork function and substantial thermionic ion emission from said ioniccathode but said heat being insuflicient to produce contact ionizationat said ionic cathode, said converter filled with a vapor with an atomicWeight greater than that of the ions emitted by said ionic cathode andan ionization potential less than that of said ions, whereby thepositive charge on said ions is transferred to the heavier atoms of saidvapor.

References Cited by the Examiner UNITED STATES PATENTS 2,510,397 6/50Hansell 310-4 2,975,320 3/61 Knauer 3104 3,021,472 2/62 Hernqvist 31043,088,851 5/63 Lemmens 313-346 3,119,059 1/64 Hall 3104 3,129,345 4/64Hatsopoulos 310-4 MILTON O. HIRSHFIELD, Primary Examiner.

L. RADER, Examiner.

1. A TWO-STAGE SPACE CHARGE NEUTRALIZED THERMIONIC CONVERTER COMPRISINGFIRST AND SECOND DISCHARGE CHAMBERS FORMED BY FIRST, SECOND AND THIRDPARALLEL ELECTRODES, SAID FIRST DISCHARGE CHAMBER FORMED BY SAID FIRSTAND SECOND ELECTRODES AS THE CATHODE AND ANODE THEREOF AND SAID SECONDDISCHARGE CHAMBER FORMED BY SAID SECOND AND THIRD ELECTRODES AS THECATHODE AND ANODE THEREOF, MEANS TO APPLY HEAT TO SAID FIRST ELECTRODE,SAID ELECTRODES HAVING PROGRESSIVELY LOWER WORK FUNCTIONS IN THEDIRECTION OF HEAT FLOW, SAID HEAT BEING SUFFICIENT TO CAUSE SUBSTANTIALTHERMIONIC ELECTRON EMISSION FROM BOTH OF SAID CATHODES, SAID SECONDELECTRODE HAVING A PLURALITY OF UNIFORMLY DISTRIBUTED BEADS OF IONEMITTING MATERIAL DEPOSITED ON EACH SURFACE THEREOF WHEREBY SAID BEADSEMIT POSITIVE ALKALI IONS FOR NEUTRALIZING SPACE CHARGE, SAID FIRSTELECTRODE HAVING A WORK FUNCTION GREATER THAN THE IONIZATION POTENTIALOF SAID ALKALI IONS, AND AN ELECTRICAL LOAD DEVICE CONNECTED EXTERNALLYBETWEEN SAID FIRST AND THIRD ELECTRODES.